Adopt `Sendable` in `NIOTestUtils` (#2199)

* Adopting `Sendable` in `NIOTestUtils`

The parameter `decoderFactory` of the static methods `ByteToMessageDecoderVerifier.verifyDecoder` do not need to be `@escaping`. I have made them non-escaping as part of `Sendable` adoption because we would otherwise need to think about if they should be `@Sendable` too.

`VerificationError` is interesting, see the code comment for more information.

* Adopting `Sendable` in `NIOTestUtils`

The parameter `decoderFactory` of the static methods `ByteToMessageDecoderVerifier.verifyDecoder` do not need to be `@escaping`. I have made them non-escaping as part of `Sendable` adoption because we would otherwise need to think about if they should be `@Sendable` too.

`VerificationError` is interesting, see the code comment for more information.

* Clarify the reason `VerificationError` already conforms to `Sendable`

Sources/NIOTestUtils/ByteToMessageDecoderVerifier.swift

@@ -19,12 +19,13 @@ public enum ByteToMessageDecoderVerifier {
     ///
     /// Verify `ByteToMessageDecoder`s with `String` inputs
     public static func verifyDecoder<Decoder: ByteToMessageDecoder>(stringInputOutputPairs: [(String, [Decoder.InboundOut])],
-                                                                    decoderFactory: @escaping () -> Decoder) throws where Decoder.InboundOut: Equatable {
+                                                                    decoderFactory: () -> Decoder) throws where Decoder.InboundOut: Equatable {
         let alloc = ByteBufferAllocator()
         let ioPairs = stringInputOutputPairs.map { (ioPair: (String, [Decoder.InboundOut])) -> (ByteBuffer, [Decoder.InboundOut]) in
             return (alloc.buffer(string: ioPair.0), ioPair.1)
         }
-        return try ByteToMessageDecoderVerifier.verifyDecoder(inputOutputPairs: ioPairs, decoderFactory: decoderFactory)
+        
+        try ByteToMessageDecoderVerifier.verifyDecoder(inputOutputPairs: ioPairs, decoderFactory: decoderFactory)
     }
 
     /// Verifies a `ByteToMessageDecoder` by performing a number of tests.
@@ -51,7 +52,7 @@ public enum ByteToMessageDecoderVerifier {
     ///     XCTAssertNoThrow(try ByteToMessageDecoderVerifier.verifyDecoder(inputOutputPairs: expectedInOuts,
     ///                                                                     decoderFactory: { ExampleDecoder() }))
     public static func verifyDecoder<Decoder: ByteToMessageDecoder>(inputOutputPairs: [(ByteBuffer, [Decoder.InboundOut])],
-                                                                    decoderFactory: @escaping () -> Decoder) throws where Decoder.InboundOut: Equatable {
+                                                                    decoderFactory: () -> Decoder) throws where Decoder.InboundOut: Equatable {
         typealias Out = Decoder.InboundOut
 
         func verifySimple(channel: RecordingChannel) throws {
@@ -207,3 +208,16 @@ extension ByteToMessageDecoderVerifier {
         }
     }
 }
+
+#if swift(>=5.5) && canImport(_Concurrency)
+/// `VerificationError` conforms to `Error` and therefore needs to conform to `Sendable` too.
+/// `VerificationError` has a stored property `errorCode` of type `ErrorCode` which can store `NIOAny` which is not and can not be `Sendable`.
+/// In addtion, `ErrorCode` can also store a user defined `OutputType` which is not required to be `Sendable` but we could require it to be `Sendable`.
+/// We have two choices:
+///  - we could lie and conform `ErrorCode` to `Sendable` with `@unchecked`
+///  - do the same but for `VerificationError`
+/// As `VerificationError` already conforms to `Sendable` (because it conforms to `Error` and `Error` inherits from `Sendable`)
+/// it sound like the best option to just stick to the conformances we already have and **not** lie twice by making `VerificationError` conform to `Sendable` too.
+/// Note that this still allows us to adopt `Sendable` for `ErrorCode` later if we change our opinion.
+extension ByteToMessageDecoderVerifier.VerificationError: @unchecked Sendable {}
+#endif

Sources/NIOTestUtils/EventCounterHandler.swift

@@ -23,7 +23,7 @@ import NIOConcurrencyHelpers
 ///
 /// - note: Contrary to most `ChannelHandler`s, all of `EventCounterHandler`'s API is thread-safe meaning that you can
 ///         query the events received from any thread.
-public final class EventCounterHandler {
+public final class EventCounterHandler: NIOSendable {
     private let _channelRegisteredCalls = NIOAtomic<Int>.makeAtomic(value: 0)
     private let _channelUnregisteredCalls = NIOAtomic<Int>.makeAtomic(value: 0)
     private let _channelActiveCalls = NIOAtomic<Int>.makeAtomic(value: 0)

Sources/NIOTestUtils/NIOHTTP1TestServer.swift

@@ -46,6 +46,11 @@ private final class BlockingQueue<Element> {
     }
 }
 
+#if swift(>=5.5) && canImport(_Concurrency)
+extension BlockingQueue: @unchecked Sendable where Element: Sendable {}
+#endif
+
+
 private final class WebServerHandler: ChannelDuplexHandler {
     typealias InboundIn = HTTPServerRequestPart
     typealias OutboundIn = HTTPServerResponsePart
@@ -303,6 +308,11 @@ extension NIOHTTP1TestServer {
     }
 }
 
+#if swift(>=5.6)
+@available(*, unavailable)
+extension NIOHTTP1TestServer: Sendable {}
+#endif
+
 // MARK: - API for HTTP server
 extension NIOHTTP1TestServer {
     fileprivate func pushChannelRead(_ state: HTTPServerRequestPart) {